1. Field of the Invention
The present invention is directed to a method and apparatus for measuring surface features of a sample with a scanning probe microscope, and more particularly, to a method and apparatus that utilize information regarding the pitch of a field of devices/features of a sample to optimize measurement performance.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip to make a local measurement of one or more properties of a sample. More particularly, SPMs typically characterize the surfaces of such small-scale sample features by monitoring the interaction between the sample and the, tip of the associated probe assembly. By providing relative scanning movement between the tip and the sample, surface characteristic data and other sample-dependent data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The atomic force microscope is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into direct or intermittent contact with a surface of the sample to be examined, and the deflection of the cantilever in response to the probe tip's interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as an arrangement of strain gauges, capacitance sensors, etc. AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
Preferably, the probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, for example, in Hansma et al. supra; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs can be designed to operate in a variety of modes, including contact mode and oscillating flexural mode. In an oscillation “flexural mode” of operation the cantilever oscillates generally about a fixed end. One popular flexure mode of operation is the so-called TappingMode™ AFM operation (TappingMode™ is a trademark of the present assignee). In a TappingMode™ AFM, the tip is oscillated flexurally at or near a resonant frequency of the cantilever of the probe. When the tip is in intermittent or proximate contact with the sample surface, the oscillation amplitude is determined by tip/surface interactions. Typically, amplitude, phase or frequency of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. These feedback signals are then collected, stored, and used as data to characterize the sample. When measuring semiconductor samples, such as the trench capacitors discussed herein, a particular version of this oscillating mode known as deep trench (DT) mode, which employs a unique and costly tip, is used.
A typical AFM system is shown in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15 is coupled to an oscillating actuator or drive 16 that is used to drive probe 14, in this case, at or near the probe's resonant frequency. Commonly, an electronic signal is applied from an AC signal source 18 under control of an AFM controller 20 to drive 16 to oscillate probe 14, preferably at a free oscillation amplitude Ao. Notably, Ao can be varied over a broad range, e.g., from microns to the nanometer scale, the latter being typically used for non-contact force measurements. As a practical matter, for low force interaction with the sample surface during imaging, Ao should be as small as possible, but large enough to prevent the tip from sticking to the sample surface 22 due to van der Waals and/or adhesive forces, for example. Probe 14 can also be actuated toward and away from sample 22 using a suitable actuator or scanner 24 controlled via feedback by computer/controller 20. Notably, the oscillating drive 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe. Moreover, though the actuator 24 is shown coupled to the probe, the actuator 24 may be employed to move sample 22 in three orthogonal directions as an X Y Z actuator.
In operation, as the probe 14 is oscillated and brought into contact with sample 22, sample characteristics can be monitored by detecting changes in the oscillation of probe 14. In particular, a deflection detection apparatus 17 a beam is directed towards the backside of probe 14 which is then reflected towards a detector 26, such as a four quadrant photodetector. As the beam translates across detector, appropriate signals are transmitted to controller 20 which processes the signals to determine changes in the oscillation of probe 14. Commonly, controller 20 generates control signals to maintain a constant force between the tip and sample, typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, As, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
As metrology applications demand greater and greater throughput, improvements when performing conventional AFM measurements have become necessary. Wafer analysis in the semiconductor industry is one key application. In general, chip makers need to measure structures having critical dimensions (CDs) that are 90 nm and below. When analyzing structures at such small scale, the corresponding measurements require uniformity control and must be able to accommodate high volume production environments. In this regard, one advancement has been in the area of automated AFMs, which greatly improve the number of samples that may be imaged in a certain time frame by minimizing expert user tasks during operation. Instruments for performing automated wafer measurements are varied but AFM offers a unique solution by providing, for example, the ability to perform high-resolution multi-dimension (e.g., 3-D) imaging. Some instruments, like the Dimension X automated AFM offered by Veeco Instruments, have proven 200 mm and 300 mm automation platforms.
Two key issues to be considered when evaluating the performance of instruments used to make such measurements include throughput and repeatability. Throughput, in this case, is the number of wafers that may be imaged per hour, and repeatability is the variation of repeated measurements made on the same object under identical conditions. A third issue concerns reproducibility, which is the variation that results when making the same measurement under different conditions. Reproducibility is important in that it determines whether the technique can accommodate condition variations when both positioning the wafer and focusing the optics. When considering these metrics, known systems have significant limitations.
To perform a semiconductor metrology experiment such as trench depth measurement of a semiconductor wafer, AFMs typically initially perform a survey scan and then perform a data scan. The survey scan acts as an aerial view of the wafer to establish a position at which the data scan may be taken. The data scan ultimately provides the information of interest (e.g., a characteristic dimension). More particularly, and as discussed in further detail below, the semiconductor wafer typically has a field of devices/features such as trenches having a location that typically is not known by the tool used for measuring the wafer. As a result, the tool must be aligned to the field to obtain quality data, and this is often accomplished with a survey scan.
In previous systems for measuring depths of semiconductor trench capacitors, the length of each scan line, spacing between adjacent scan lines (i.e., resolution or density), and number of scan lines is user-defined. For example, automated AFMs typically take a large number of lines of data (i.e., high density), for example, 32 or 48 lines each having a length of typically at least two times the pitch of the features over a set region to obtain a high resolution survey image. Typically, the pitch of a field of features has at least two components, one in “X” and one in “Y”, for the 2-D array. The pitch is the distance in each direction in which the pattern of features repeats itself. The length and height of the scan may range from about 1 micron to 50 microns or more.
Thereafter, a pattern recognition operation is conducted to identify a pair of the features and thereby establish a center for the data scan. Then, a data scan at high resolution is performed using a zig-zag (i.e., raster) scan, typically around the midpoint of the identified trenches. Moreover, according to some techniques, multiple survey scans may be conducted; for example, one technique uses a first survey scan in “X” (typically part of a “searching” routine) and then performs a second survey scan in “Y” to establish the center. Overall, these known processes require a significant amount of time to complete, given the multitude of scans (both survey and data), and thus severely compromise throughput. When considering that the scans are conducted at high resolution, the limitation concerning poor throughput is only exacerbated.
Notably, other drawbacks also concern the high resolution of the scans. For instance, because the survey scan of these known techniques is a high density scan, the scan is often directed to a small feature set, e.g., one, or a pair, of surface feature(s). This small population yields conclusions that are less than ideal. For instance, as understood, pattern recognition programs provide superior output the larger the population of data. As a result, relatively small feature sets used in the prior art can severely adversely impact the quality of the measurement. Another drawback associated with high density scans is that the probes and corresponding tips are highly precise and costly components which need to be replaced after significant wear. By conducting such high resolution scans, tip life can be drastically reduced, a significant drawback considering that the tips may cost $1000 or more. A still further drawback is that high resolution scans take a long time to complete, as one would expect. Not only is this a limitation in and of itself, it leads to other problems, including making environmental factors more problematic. For example, measurement repeatability can be significantly reduced given that thermal drift is greater the more time it takes to perform a measurement.
Moreover, a further problem with known techniques is that when, for example, measuring the depth of the features, the data acquired over the series of scans is averaged. Noting that the maximum depth is often a key metric to be determined, this averaging of the data hinders the systems ability to identify maximum depth given that depth measurements less than the maximum are “averaged” along with, typically, the maximum of the series of scans. As a result, such methods typically undesirably underestimate trench depth.
As a result, what was desired in the field of automated AFM measurements, particularly in the semiconductor industry, was a method and system able to perform both survey and data scans with a minimum amount of lines over a broad range of user-selected scan lengths (e.g., to analyze a larger number of features/devices) without compromising measurement performance. Improved throughput and repeatability, as well as improved tip wear performance was ultimately needed.